The present invention relates generally to gamma ray imaging detectors and more particularly to a gamma ray camera having application in the field of nuclear medicine for providing spatial images of radioactive substances administered into the body to elucidate internal organ functions.
In nuclear medicine practice, gamma emitting radioisotope containing substances may be introduced into an area of the body under examination, such as the thyroid, and may be localized preferentially into a well defined distribution depending on the chemical and biological activity of the substance. Gamma rays are emitted isotropically from the substance at energies characteristic of the isotope. For example, technicium (99 m) generates gamma rays at about 140 Key and, because of its relatively short half-life (&lt;1 day), is used in many nuclear medicine applications. Because radiation from the substance is isotropic, the gamma rays must be collimated in order to obtain an image of the region of the body into which the radioisotope is introduced.
Detecting gamma ray photons in general requires a high gain process to convert a single photon impact into many free electrons that in turn register an electrical impulse in a counter. For this purpose, Anger (U.S. Pat. Nos. 2,776,377; 2,779,876) used scintillation properties of certain crystals and the high gain provided by photomultiplier tubes; the collimator is coupled to a scintillator crystal through a light tight thin barrier that passes gamma photons but blocks visible photons. The scintillation crystal (typically sodium iodide (NaI)) converts a gamma photon (having energies generally greater than about 40 KeV) into a number of visible photons (wavelengths of about 200-1000 nm) proportional to the gamma photon energy. The location of scintillation points is detected by the photomultiplier tube array coupled to the crystal through the light pipe. The whole system is configured in a hexagonal close packed arrangement such that the signal from a given scintillation event excites responses in at least seven photomultiplier tubes, i.e. a central tube and at least the six nearest tubes. Anger combined the magnitude of the signal from each photomultiplier tube to provide a two dimensional position signal indicative of the point of gamma ray absorption. This process required a large electronic system to compute all the coincidental responses from a large set of combinations of typically 72 elements taken 7 at a time.
Proposed improvements in recent years for overcoming the limitations of the Anger camera. have included use of intensifier tubes, position sensing photomultiplier tubes, and arrays of solid state photodetectors, all with very limited success. The intensifier tube proposals have limited sensitivity and resolution because of insufficient gain and high background noise. An example of this type of camera is one proposed by Lo I. Yin (1979), in which the converting scintillation crystal is coupled directly to a fiberoptic faceplate which is coupled to an image intensifier tube. This camera is limited by insufficient gain in the photodetector to cover a large area, and the photodetector must be scaled up to the size of the input crystal to achieve efficient light coupling. More recently, Dilmanian et al (IEEE transactions on Nuclear Science, 37 (1990)) described a gamma imaging system comprising an NaI crystal coupled by a fiberoptic taper to the entrance port of a Hamamatsu microchannel-plate imager (PIAS) photon counting imaging system. Although the imaging system demonstrated an autoradiograpic resolution of about 0.7 mm FWHM, it was limited in cross-sectional dimension to 26 mm. To scale this instrument up to Anger camera dimensions would be prohibitively expensive since conventional imagers have an active diameter of only 15 mm. A factor affecting the Dilmanian et al and similar imagers is that there is no pulse height selection on the detected gamma photon distribution events. This will result in degraded imaging of deep tissue due to Compton scattering, which confounds the gamma isotope distribution image.
An example solid state detector array type of gamma imager by Engdahl et al (U.S. Pat. No. 5,171,998) includes the usual front end gamma scintillator followed by an array of solid state photo-diode detectors, preferably low capacitance silicon drift elements. The impact of the gamma ray photon is tracked in two dimensions by a complex timing scheme on two coordinates. Although small arrays and imaging devices show some promise, limitations arise when making these arrays much larger. Because the photodiodes as described by Engdahl et al are to be directly mounted to the scintillator crystal or light pipe attached to it, there is the possibility of gamma ray photons striking directly on the photodiodes. This could have the effect of creating an intense background event structure which would be difficult to compensate.
Two limitations of the original Anger camera addressed by recent inventions include the inability of the Anger camera to distinguish pulse height and use of NaI as the scintillator. The first limitation does not permit the camera to reject multiple scatter gamma photons which confounds the isotope distribution image. The second is the use of NaI crystal for its blue spectrum center of its scintillation photons. This blue spectrum center was selected for optimum use of photomultiplier tubes which are more sensitive in the blue, although the visible photon yield is notably lower than that of other scintillator crystals. Also, NaI is fragile and hydroscopic, requiring careful handling.
The present invention solves or substantially reduces in critical importance problems with prior art camera structures by providing an inexpensive gamma camera which uses any of several well known gamma scintillation detector crystals such as thallium doped NaI or thallium doped cesium iodide (CsI). The increased photon yield of CsI(T1), although further into the red spectrum, is a desirable attribute, and CsI is less fragile and easier to handle than NaI and is not hydroscopic. The invention further utilizes a novel high gain, low noise, high quantum efficiency intensifier system combined with a video signal processor to produce images of radioisotope distributions in the body.
It is therefore a principal object of the invention to provide an improved gamma imaging camera system.
It is a further object of the invention to provide a gamma imaging camera utilizing a scintillation crystal that permits retrofit to existing camera systems.
It is a further object of the invention to provide a gamma imaging camera system permitting use of CsI(Tl) as the scintillator crystal which is less fragile, less hydroscopic and has greater photon yield than NaI(Tl).
It is a further object of the invention to provide an economical and portable gamma camera having no light pipes for coupling the scintillator crystals and the photon detector.
It is another object of the invention to provide a gamma imaging camera incorporating a high efficiency coupling lens to capture the scintillation photons from a large area crystal and couple the signal to a small aperture sensor system.
It is a further object of the invention to utilize night vision inverter tube assemblies coupled and operated in a novel manner.
It is another object of the invention to provide a gamma camera having ultra low level detection capability with a gain of about 10.sup.9 with noise less than 10.sup.-3 /pixel/sec.
It is another object of the invention to provide a gamma imaging camera wherein the image processing is simplified by using conventional video camera output with a VCR and personal computer control.
It is a further object of the invention to provide a gamma imaging camera incorporating Compton scatter rejection in the gamma image by thresholding in the image processing.
It is another object of the invention to provide a gamma imaging camera incorporating cosmic ray background rejection by band limiting of the scintillation events in the image processing.
It is yet another object of the invention to provide an ultra-low level (essentially single-photon) visible spectrum imaging camera system.
These and other objects of the invention will become apparent as a detailed description of representative embodiments thereof proceeds.